Optical lithography is an optical exposure process which transfers the pattern of the mask plate onto the silicon wafer. According to different exposure methods, Optical lithography is divided into contact lithography, proximity lithography, step-and-repeat projection lithography, and step-and-scan projection lithography. Step-and-scan projection lithography may effectively increase the chip productivity, therefore it has become a mainstream lithography technology.
The illumination system is one of the key parts for a lithographic projection exposure step-and-scan apparatus. In order to precisely replicate various fine patterns onto the silicon wafer, the illumination system is required to form an illumination field with variable coherence factor and uniform intensity distribution, while achieving size adjustable by scanning the illumination field continuously.
U.S. Pat. No. 8,004,656B2 discloses an illumination system for lithographic projection exposure step-and-scan apparatus, which includes a light source, a beam expansion unit, a zoom lens group, a pair of axicon elements, a field defining component, a condenser lens, a reticle masking unit, a rema objective, and a reticle plane. Light emitted from the light source passes through the beam expansion unit, the zoom lens group, and a pair of axicon elements sequentially, becomes a beam distribution with specific illumination pattern and coherent factors, then is homogenized by the field defining unit and a condenser lens group, uniform intensity distribution field is formed at the reticle masking unit, finally imaged onto the mask by rema objective. The reticle masking unit plane is conjugate to the mask plane, so scanning reticle masking unit plane may scan the illumination field on the mask simultaneously. The structure of the illumination system is complex, the diameter of the condenser lens is about several hundred millimeters. And it requires a high scanning speed of the scanning slit, as high as up to several hundred millimeters per second, or even a few meters per second.
U.S. Patent Publication No. 2009/0257041A1 discloses another illumination system for lithographic projection exposure step-and-scan apparatus, the illumination system includes a source, a zoom, axicons, the array, a shutter plate, a condenser, and a mask. Light emitted from source is focused on the plane of the shutter plate by the array, enlarged by the condenser, and then superimposed on the mask. Means drives the shutter plate to move to scanning the illumination field on the mask. Compared with U.S. Pat. No. 8,004,656 B2, the system requires a lower scanning speed, but because the size of arrays is small, it needs up to micron even submicron precision during scanning.